Large Quadratic Nonlinear Optical Efficiencies in Pseudosymmetric Streptocyanine Dyes

The quadratic (∝ E 2 ) nonlinear optical (NLO) properties of pseudosymmetric organic cations are investigated. These chromophores belong to a family of highly polarizable streptocyanine dyes built in two steps by amination of their related carboxonium salts. They exhibit a nearly perfect optical transparency in the 200−1100 nm range, except around 700 nm, where a single but narrow band is present. Some of these dyes crystallize in noncentrosymmetric space groups, which, despite a symmetrical π-electronic structure, leads to symmetry lowering and restores a push−pull character to the optical transition and hence a sizable molecular hyperpolarizability. One of these molecules is optimized for a solid-state NLO effect and exhibits an efficiency 120 times that of urea in second harmonic generation at 1.907 μm.